Population connectivity – the degree to which spatially discrete
subpopulations supply, receive, or exchange larvae – is a fundamental
feature of the dynamics of marine organisms. Connectivity rates
determine colonization patterns of new habitats, the resiliency of
populations to disturbances or harvesting, and should be used to inform
the design of marine protected areas (MPAs). Quantifying rates of
exchange in marine organisms, is, unfortunately, extremely difficult
because the natal origin of most adults is almost always unknown. This
lack of knowledge is primarily due to the reproductive strategy
employed by most marine species. Typically, this involves the
production of large numbers of very small pelagic larvae that suffer
high initially mortality rates, precluding traditional mark-recapture
approaches. Although various studies have attempted to resolve
questions of larval connectivity using models and population genetics,
very little empirical evidence is available to test the scenarios
generated by these studies. Managers are thus forced to make decisions
about MPA designs without the critical knowledge of whether or not
these designs will work effectively.

We have recently used innovative multi-disciplinary techniques to
directly measure population connectivity in two fish species in Kimbe
Bay, Papua New Guinea, where an extensive network of MPAs have been
established. Using a chemical tagging method, we found that 60% of baby
fish of both study species on our reef were offspring of parents on
that same reef (known as selfrecruitment). This provided, for the first
time, a direct measurement of connectivity in coral reef fish and
demonstrated that it is possible to obtain this much-needed empirical
evidence.

The second technique we employed to assess connectivity is genetic
parentage analysis. While conducting our tagging study, we took genetic
samples from individuals of both our study. We have now sufficiently
refined genetic analyses to match individual offspring to the exact
father and mother in our populations, providing us with high-resolution
information about the patterns of reproductive output and larval
dispersal. This opens the door for us to ask new questions: Do some
parents produce more successful offspring than others? Can we identify
areas of reefs that are more productive than others? For how many
breeding seasons are individuals contributing to future generations?
These questions, while of great significance, have been previously
unanswerable due to the constraints described above as well as a lack
of resolution in genetic methods.

To date, we have conducted the chemical tagging analyses for both of
our study species, the orange clownfish (Amphiprion percula, of Finding
Nemo fame) and the vagabond butterflyfish (Chaetodon vagabundus). The
genetic parentage analysis has subsequently been completed for the
clownfish, and it is important to note that there was 100% agreement
between the two techniques. However, clownfish and butterflyfish have a
major difference in their reproductive behavior. Clownfish lay their
eggs on the actual surface of the reef. After hatching, clownfish
larvae are planktonic for about 10 days before they settle back to a
reef and begin the juvenile stage of their lives. About 75% of reef
fish species (including butterflyfish and all commercially important
species) do not lay their eggs on the substrate, but rather release
their eggs and sperm into the water column, where the eggs will drift
with the currents for several days before hatching. Once these
pelagically spawned eggs hatch, they typically spend 4-6 weeks (5 weeks
for butterflyfish) in their pelagic stage before settling onto a reef.
As this presents the potential for pelagically spawned larvae to
disperse over great distances, it is significant that we found
identical levels of self-recruitment in both species.

The present proposal seeks funds to complete the genetic parentage
analyses on the samples from the vagabond butterflyfish that we have
already collected as a part of our study. The potential to demonstrate
that parentage analysis can be applied as a tool for determining
connectivity in pelagic-spawning reef fish represents an opportunity to
answer important fundamental questions of reef fish dynamics with clear
management implications.

Last updated: May 23, 2008

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